1. Field of the Invention
The present invention relates to a variable nozzle unit which can change a passage area for (a flow rate of) an exhaust gas to be supplied to a turbine impeller side in a variable-geometry turbocharger, and the like.
2. Description of the Related Art
A typical variable nozzle unit used in a variable-geometry turbocharger is disposed between a turbine scroll passage and a gas discharge port inside a turbine housing in such a way as to surround a turbine impeller. A specific configuration of such a typical variable nozzle unit (a conventional variable nozzle unit) is as follows (see Japanese Patent Application Laid-Open Publication No. 2006-125588 (FIG. 9 and FIG. 10)).
A nozzle ring is disposed in the turbine housing. As shown in FIG. 6A and FIG. 6B, a shroud ring 157 is provided integrally with the nozzle ring (not shown) at a position away from and opposed to the nozzle ring in an axial direction of a turbine impeller 129. Meanwhile, the shroud ring 157 includes a cylindrical shroud portion 163 which is placed on an inner peripheral edge side, which projects to the gas discharge port side (a downstream side), and which covers outer edges of multiple turbine blades 133 of the turbine impeller 129. In addition, the shroud portion 163 of the shroud ring 157 is placed inside of an annular step portion 141 formed on an inlet side of the gas discharge port inside the turbine housing. A ring groove 165 is formed in an outer peripheral surface of the shroud portion 163 of the shroud ring 157.
Multiple variable nozzles (not shown) are disposed at regular intervals in a circumferential direction between opposed surfaces of the nozzle ring (not shown) and the shroud ring 157. Each variable nozzle is turnable in forward and reverse directions (opening and closing directions) about its shaft center which is in parallel with a shaft center Z of the turbine impeller 129. Here, when the multiple variable nozzles are synchronously turned in the forward direction (the opening direction), a passage area for an exhaust gas to be supplied to the turbine impeller 129 side is increased. On the other hand, when the multiple variable nozzles are synchronously turned in the reverse direction (the closing direction), the passage area for the exhaust gas is decreased.
Multiple seal rings (an upstream-side seal ring 183 and a downstream-side seal ring 185) are provided in pressure-contact, by their own elastic forces, with an inner peripheral surface of the step portion 141 of the turbine housing. The multiple seal rings 183 and 185 suppress leakage of the exhaust gas from the turbine scroll passage side. Meanwhile, inner peripheral edge portions of the seal rings 183 and 185 are fitted into the ring groove 165 of the shroud ring. Here, a circumferential position of an end gap 183f of the upstream-side seal ring 183 is displaced from a circumferential position of an end gap 185f of the downstream-side seal ring 185.
Note that FIG. 6A is a view taken along the VIA-VIA line in FIG. 6B, and FIG. 6B is a view showing part of the conventional variable nozzle unit. In the drawings, “L” indicates leftward and “R” indicates rightward.